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https://issues.apache.org/jira/browse/FLINK-33962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17815195#comment-17815195
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Zhanghao Chen commented on FLINK-33962:
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Hi [~srichter], I'm trying to make the hasher upgrade backwards-compatible by
reviving the idea of FLINK-5290 authored by you back in Flink 1.2. The details
can be found in the linked FLIP doc. Could you kindly help take a review?
Thanks a lot in advance.
> Chaining-agnostic OperatorID generation for improved state compatibility on
> parallelism change
> ----------------------------------------------------------------------------------------------
>
> Key: FLINK-33962
> URL: https://issues.apache.org/jira/browse/FLINK-33962
> Project: Flink
> Issue Type: Improvement
> Components: API / Core
> Reporter: Zhanghao Chen
> Priority: Major
>
> *Background*
> Flink restores opeartor state from snapshots based on matching the
> operatorIDs. Since Flink 1.2, {{StreamGraphHasherV2}} is used for operatorID
> generation when no user-set uid exists. The generated OperatorID is
> deterministic with respect to:
> * node-local properties (the traverse ID in the BFS for the stream graph)
> * chained output nodes
> * input nodes hashes
> *Problem*
> The chaining behavior will affect state compatibility, as the generation of
> the OperatorID of an Op is dependent on its chained output nodes. For
> example, a simple source->sink DAG with source and sink chained together is
> state imcompatible with an otherwise identical DAG with source and sink
> unchained (either because the parallelisms of the two ops are changed to be
> unequal or chaining is disabled). This greatly limits the flexibility to
> perform chain-breaking/joining for performance tuning.
> *Proposal*
> Introduce {{StreamGraphHasherV3}} that is agnostic to the chaining behavior
> of operators, which effectively just removes L227-235 of
> [flink/flink-streaming-java/src/main/java/org/apache/flink/streaming/api/graph/StreamGraphHasherV2.java
> at master · apache/flink
> (github.com)|https://github.com/apache/flink/blob/master/flink-streaming-java/src/main/java/org/apache/flink/streaming/api/graph/StreamGraphHasherV2.java].
>
> This will not hurt the deteministicity of the ID generation across job
> submission as long as the stream graph topology doesn't change, and since new
> versions of Flink have already adopted pure operator-level state recovery,
> this will not break state recovery across job submission as long as both
> submissions use the same hasher.
> This will, however, break cross-version state compatibility. So we can
> introduce a new option to enable using HasherV3 in v1.19 and consider making
> it the default hasher in v2.0.
> Looking forward to suggestions on this.
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